The various embodiments relate to the field of marine electromagnetic survey methods and related systems. More specifically, various embodiments relate to electromagnetic survey streamer cables that can be electrically reconfigured to have selectable receiver spacing and offset.
Marine controlled source electromagnetic (CSEM) surveying is a geophysical surveying technique that uses electromagnetic (EM) energy to identify possible hydrocarbon bearing rock formations below the bottom of a body of water such as a lake or the ocean. In a typical marine CSEM survey, an EM source and a number of EM sensors are located within a body of water. The EM source is typically towed over an area of interest in the Earth's subsurface, and the sensors located over the area of interest to obtain signals related to the distribution of electrical resistivity in the subsurface area of interest. Such surveying may be performed for a range of EM source and EM sensor positions. The EM source emits either or both a time varying electric field and a time varying magnetic field, which propagate outwardly into the overlying seawater and downwardly into the formations below the water bottom. The sensors may detect and record the induced electric field. The time varying EM field may be induced by passing electric current through an antenna. The electric current may be continuous wave and have one or more discrete frequencies. Such current passing through an antenna is used for what is referred to as “frequency domain CSEM” surveying. In other situations, direct current is applied to an antenna, and transient EM fields are produced by switching the current. Such switching may include, for example, switching on, switching off, inverting polarity, and inverting polarity after a switch-on or switch-off event. Such switching may be sequenced in time, for example, equally time spaced, or in a time series known as a “pseudo random binary sequence.” Such switched current is used to conduct what is referred to as a “transient CSEM” survey.
The EM energy is rapidly attenuated in the conductive seawater, but in less conductive subsurface formations is attenuated less and propagates more efficiently. If the frequency of the EM energy is low enough, the EM energy can propagate deep into the subsurface formations. Energy “leaks” from resistive subsurface layers, e.g., a hydrocarbon-filled reservoir, back to the water bottom. When the source-sensor spacing (“offset”) is comparable to or greater than the depth of burial of the resistive layer (the depth below the water bottom) the energy reflected from the resistive layer will dominate over the transmitted energy. CSEM surveying uses the large resistivity contrast between highly resistive hydrocarbons and conductive aqueous saline fluids disposed in permeable subsurface formations to assist in identifying hydrocarbon reservoirs in the subsurface.
The sensor layout in an electromagnetic survey system may consist of spaced apart electrode pairs distributed along the length of the streamer cable. The electrode separation may increase as a function of offset to the electromagnetic source, thus the hardware configuration is changed based on the absolute position at which the measurement node is located.
There continues to be a need for improved configurations of electromagnetic sensor streamer cable that simplify construction and reduce production of unique parts for cost control.